Concept of Movement of Substances Across a Plasma Membrane

3.2

Concept of Movement of Substances Across a Plasma Membrane

Characteristics of Substances That Can Across The Plasma Membrane

There are three common factors that determine whether a molecule can pass through a plasma membrane, which are molecule size, polar molecule and ionic charge

Movement of Substances Across Plasma Membrane

The movement of substances across a plasma membrane occurs through passive and active transport

Passive Transport

Simple Diffusion

Simple diffusion is the movement of molecules or ions from an area of high concentration to an area of low concentration
The moving molecules are said to move down the concentration gradient until a dynamic equilibrium is achieved
This may occur with or without the presence of a plasma membrane
Lipid soluble molecules (fatty acids and glycerol), oxygen and carbon dioxide diffuse through the phospholipid bilayer as simple diffusion

Osmosis

Osmosis is a passive transport process that is similar to diffusion but it involves only water molecules
Osmosis refers to the net movement of water molecules from an area of high water potential (low solutes concentration) to an area of low water potential (high solutes concentration) randomly through a selectively permeable membrane
The selectively permeable membrane is permeable to water but impermeable to some solutes such as sucrose molecules
The same situation occurs in cells through the phospholipid bilayer

Facilitated Diffusion

Lipid-insoluble molecules such as ions, large molecules such as amino acids and glucose are unable to pass through the phospholipid bilayer
These substances move across the membrane with the aid of transport proteins (carrier or channel proteins)
- Channel protein: form channels or canals to allow small-sized solutes and ions to diffuse across the plasma membrane.
- Channels have specific internal characteristics that only allow specific ions to pass through it
- Carrier protein: have specific sites and can only bind to a specific molecule
- For example, glucose molecules can only bind to the specific site of a glucose carrier protein
This process is known as facilitated diffusion
Facilitated diffusion does not require energy because the transport proteins transport molecules down a concentration gradient
The process continues until a dynamic equilibrium is achieved when the concentration of molecules is the same at both sides of membranes

Active Transport

Movement of substances across a plasma membrane may occur by active transport
There is another type of pump known as the + proton pump
For example, the proton pumps that are found on the epithelial cells lining the stomach cavity
Proton pump causes the acidity of the stomach contents
Energy from the ATP enables the hydrogen ion to be transported by the carrier proteins (proton pump) towards the extracellular fluid
This causes an accumulation of the hydrogen ion and acid production in the stomach cavity
The characteristics of an active transport are as follows:
- The movement of molecule or ion substances across a plasma membrane occurs against a concentration gradient It requires energy from ATP (adenosine triphosphate) molecules generated during cellular respiration
- It requires specific carrier protein with specific sites to bind with certain molecules or ions
- Carrier proteins also possess receptors to bind with ATP molecules
- Carrier proteins change shape when a phosphate group attaches to it. As a result, molecules or ions move across a membrane